1. Field of the Invention
The present invention relates to sealing glass pastes, and, in particular, to sealing glass pastes for cathode ray tubes (CRT). Cathode ray tubes are fabricated by sealing together a glass faceplate or panel supporting the phosphorescent display screen of the tube and a glass funnel having an electrically conductive interior coating which forms part of the electronic circuitry of the tube. To seal the components together a sealing glass paste is applied to the edges of one part (either the face panel or the funnel) in the form of a ribbon, which is then dried. The remaining part is then placed in contact with the dried paste and the assembly is heated to evaporate the solvent, burn out the binder and fuse the sealing glass.
2. Description of the Related Art
Wet coating techniques for sealing glass or ceramic parts have been proposed and generally have involved pastes having a sealing glass distributed throughout a binder/solvent system in which the solvent is an organic solvent. When the assembly has been formed it is then heated to evaporated the solvent, burnout the binder and fuse the glass. For example, U.S. Pat. No. 4,014,845 discloses a binder/solvent system which comprises a polymeric material which is a polymer containing oxygen and a monomer of an alkyl methacrylate and a solvent for the polymeric material. For sealing glasses which contain PbO and which melt and flow at low temperatures, preferably at below 475.degree. C., such as PBO--B.sub.2 O.sub.3 --ZnO, the binder/solvent system which has been used commercially comprises nitrocellulose/amyl acetate or similar esters. Nitrocellulose has many disadvantages for use in such systems, but it has been tolerated because it does not cause the dielectric breakdown of the sealing glasses caused by reduction of PbO to Pb.
Other binders which have been tried in place of nitrocellulose do not prevent the reduction of PbO in the sealing glass to Pb metal during the sealing process. Thus, instead of a yellow seal being formed by the devitrified solder glass, a grey or grey-black seal is formed, which indicates the presence of metallic lead. Some of the discoloration may also be due to carbon residues formed on incomplete burn-out of the binder.
U.S. Pat. No. 3,973,975 discloses that the incorporation of Pb.sub.3 O.sub.4 in the sealing glasses containing PbO and the pastes made therewith, prevents the reduction of PbO to Pb. The Pb.sub.3 O.sub.4 is reduced to PbO, but the additional amount of PbO so formed does not affect the properties of the seal. The Pb.sub.3 O.sub.4 also acts to suppress the chemical reduction of the sealing glass by organic vapors during the sealing step.
U.S. Pat. No. 4,154,494 disclosed a method of preventing the chemical reduction of the devitrifiable solder glass by providing an oxygen-evolving agent with the bulb during the sealing step. The oxygen-containing compound is coated onto the funnel in an amount sufficient to suppress chemical reduction of the devitrifiable solder glass during sealing. Thus, the seal resists dielectric breakdown when very high voltages are subsequently applied.